In recent years, the security consciousness has been raised due to worsening security. In particular, physical security has been introduced into various facilities of not only large-scale facilities such as airports or power plants but also general enterprises, commercial facilities or public institutions. Although entering-leaving management at the gate of the facility has been a mainstream regarding the conventional physical security, surveillance intended for whole site of the facility becomes a mainstream lately. As conventional systems for detecting an intruder who intrudes into a predetermined surveillance region to be guarded, the intrusion detection system described in Patent Document 1 and the object detection apparatus described in Patent Document 2 have been known.
The intrusion detection system described in the Patent Document 1 is characterized by including a plurality of antennas installed in a detection region, a transmitter that transmits a signal from one of the plurality of antennas, a receiver that detects signals received by the other antennas, a calculator that detects amounts of changes in the signals detected by the receiver, and a judging device that judges whether or not an intrusion into the detection region has occurred based on the amounts of changes. In this case, the judging device judges that an intrusion into the detection region has occurred when at least one of the change in the amplitude of the signal and the change in the phase of the signal that are detected by the calculator is equal to or larger than a predetermined value.
In addition, the object detection apparatus described in the Patent Document 2 is characterized by including a transmitting cable, a receiving cable, a transmitter part connected to the transmitting cable to transmit a high-frequency current to the transmitting cable, and a receiver part connected to the receiving cable. The object detection apparatus receives electromagnetic waves transmitted from the transmitting cable by the receiving cable, and detects the presence or absence of an object based on a change in the intensity of the electromagnetic waves received by the receiving cable. In this case, the transmitter part includes means for changing standing waves generated in the transmitting cable. Concretely speaking, the object detection apparatus described in the Patent Document 2 judges that an intruder has passed over the receiving cable laid underground when it is detected by the receiver part that the amount of decrease in the received current intensity has exceeded a predetermined threshold value.